Matter that remained following the initial expansion has since undergone gravitational collapse to create stars, galaxies and other astronomical objects, leaving behind a deep vacuum that forms what is now called outer space. When this happened, matter and energy became decoupled, allowing photons to travel freely through the continually expanding space. About 380,000 years later the Universe had cooled sufficiently to allow protons and electrons to combine and form hydrogen-the so-called recombination epoch. According to the Big Bang theory, the very early Universe was an extremely hot and dense state about 13.8 billion years ago which rapidly expanded. The size of the whole universe is unknown, and it might be infinite in extent. Given the finite speed of light, this view covers the past 13 billion years of the history of outer space. Part of the Hubble Ultra-Deep Field image showing a typical section of space containing galaxies interspersed by deep vacuum. Microgravity has a negative effect on human physiology that causes both muscle atrophy and bone loss. Outer space represents a challenging environment for human exploration because of the hazards of vacuum and radiation. On the other hand, uncrewed spacecraft have reached all of the known planets in the Solar System. The economic cost of putting objects, including humans, into space is very high, limiting human spaceflight to low Earth orbit and the Moon. This was followed by crewed rocket flights and, then, crewed Earth orbit, first achieved by Yuri Gagarin of the Soviet Union in 1961. Humans began the physical exploration of space during the 20th century with the advent of high-altitude balloon flights. Despite the drafting of UN resolutions for the peaceful uses of outer space, anti-satellite weapons have been tested in Earth orbit. This treaty precludes any claims of national sovereignty and permits all states to freely explore outer space. The framework for international space law was established by the Outer Space Treaty, which entered into force on 10 October 1967. Certain portions of the upper stratosphere and the mesosphere are sometimes referred to as "near space". The Kármán line, an altitude of 100 km (62 mi) above sea level, is conventionally used as the start of outer space in space treaties and for aerospace records keeping. Outer space does not begin at a definite altitude above Earth's surface. Most of the remaining mass-energy in the observable universe is made up of an unknown form, dubbed dark matter and dark energy. Intergalactic space takes up most of the volume of the universe, but even galaxies and star systems consist almost entirely of empty space. Local concentrations of matter have condensed into stars and galaxies. The plasma between galaxies is thought to account for about half of the baryonic (ordinary) matter in the universe, having a number density of less than one hydrogen atom per cubic metre and a kinetic temperature of millions of kelvins. The baseline temperature of outer space, as set by the background radiation from the Big Bang, is 2.7 kelvins (−270 ☌ −455 ☏). Outer space is not completely empty it is a near-perfect vacuum containing a low density of particles, predominantly a plasma of hydrogen and helium as well as electromagnetic radiation, magnetic fields, neutrinos, dust, and cosmic rays. Outer space, commonly referred to simply as space, is the expanse that exists beyond Earth and its atmosphere and between celestial bodies. The layers of the atmosphere are drawn to scale, whereas objects within them, such as the International Space Station, are not. The Kármán line at an altitude of 100 km (62 mi) is shown. The interface between Earth's surface and outer space.
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